Process Capability Evaluation - Looking beyond the obvious
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چکیده
Process Capability: Definition and standard evaluation: Process Capability tells you, to what extent the process is likely to satisfy the customer's wish. Standard Process Capability calculations assume 99.73% coverage of the process mapped as σ 3 ± x. Where x is the process mean σ is the process standard deviation. The standard evaluation of Process Capability Indices Cp & Cpk is given by the following formulae:) , (_ 3 3 6 Lower pk Upper pk pk Lower pk Upper pk p C C of Minimum C LSL x C x USL C LSL USL C = − = − = − = σ σ σ Where USL =Upper Specification Limit LSL = Lower Specification Limit x = Process mean σ =Process standard deviation This article touches on basic Process Capability calculations with bilateral as well as unilateral (single-sided) tolerances. It further discusses various approaches that may be taken when the data are not normally distributed. Some of the more common data transformation techniques such as Box-Cox and Johnson transforms are discussed. Clements' method for analyzing non-normal data by fitting Pearson Curves is also explained.
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